Maintainer	mainland@eecs.harvard.edu
Safe Haskell	None

Control.Monad.Exception

Description

Synopsis

Documentation

class (Typeable e, Show e) => Exception e where

Any type that you wish to throw or catch as an exception must be an instance of the Exception class. The simplest case is a new exception type directly below the root:

 data MyException = ThisException | ThatException deriving (Show, Typeable) instance Exception MyException

The default method definitions in the Exception class do what we need in this case. You can now throw and catch ThisException and ThatException as exceptions:

 *Main> throw ThisException `catch` \e -> putStrLn ("Caught " ++ show (e :: MyException)) Caught ThisException

In more complicated examples, you may wish to define a whole hierarchy of exceptions:

 --------------------------------------------------------------------- -- Make the root exception type for all the exceptions in a compiler data SomeCompilerException = forall e . Exception e => SomeCompilerException e deriving Typeable instance Show SomeCompilerException where show (SomeCompilerException e) = show e instance Exception SomeCompilerException compilerExceptionToException :: Exception e => e -> SomeException compilerExceptionToException = toException . SomeCompilerException compilerExceptionFromException :: Exception e => SomeException -> Maybe e compilerExceptionFromException x = do SomeCompilerException a <- fromException x cast a --------------------------------------------------------------------- -- Make a subhierarchy for exceptions in the frontend of the compiler data SomeFrontendException = forall e . Exception e => SomeFrontendException e deriving Typeable instance Show SomeFrontendException where show (SomeFrontendException e) = show e instance Exception SomeFrontendException where toException = compilerExceptionToException fromException = compilerExceptionFromException frontendExceptionToException :: Exception e => e -> SomeException frontendExceptionToException = toException . SomeFrontendException frontendExceptionFromException :: Exception e => SomeException -> Maybe e frontendExceptionFromException x = do SomeFrontendException a <- fromException x cast a --------------------------------------------------------------------- -- Make an exception type for a particular frontend compiler exception data MismatchedParentheses = MismatchedParentheses deriving (Typeable, Show) instance Exception MismatchedParentheses where toException = frontendExceptionToException fromException = frontendExceptionFromException

We can now catch a MismatchedParentheses exception as MismatchedParentheses, SomeFrontendException or SomeCompilerException, but not other types, e.g. IOException:

 *Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses)) Caught MismatchedParentheses *Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException)) Caught MismatchedParentheses *Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException)) Caught MismatchedParentheses *Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: IOException)) *** Exception: MismatchedParentheses

Methods

toException :: e -> SomeException

fromException :: SomeException -> Maybe e

data SomeException

The SomeException type is the root of the exception type hierarchy. When an exception of type e is thrown, behind the scenes it is encapsulated in a SomeException.

Instances

Show SomeException
Typeable SomeException
Exception SomeException

class Monad m => MonadException m whereSource

Methods

throw :: Exception e => e -> m aSource

Throw an exception.

catch Source

Arguments

:: Exception e
=> m a	The computation to run
-> (e -> m a)	Handler to invoke if an exception is raised
-> m a

Catch an exception.

finally Source

Arguments

:: m a	The computation to run
-> m b	Computation to run afterward (even if an exception was raised)
-> m a

Run a computation and always perform a second, final computation even if an exception is raised. If a short-circuiting monad transformer such as ErrorT or MaybeT is used to transform a MonadException monad, then the implementation of finally for the transformed monad must guarantee that the final action is also always performed when any short-circuiting occurs.

Instances

MonadException IO
MonadException STM
(Monad (MaybeT m), MonadException m) => MonadException (MaybeT m)
(Monad (ListT m), MonadException m) => MonadException (ListT m)
(Monad (IdentityT m), MonadException m) => MonadException (IdentityT m)
(Monad (ExceptionT m), Monad m) => MonadException (ExceptionT m)
(Monad (WriterT w m), Monoid w, MonadException m) => MonadException (WriterT w m)
(Monad (WriterT w m), Monoid w, MonadException m) => MonadException (WriterT w m)
(Monad (StateT s m), MonadException m) => MonadException (StateT s m)
(Monad (StateT s m), MonadException m) => MonadException (StateT s m)
(Monad (ReaderT r m), MonadException m) => MonadException (ReaderT r m)
(Monad (ErrorT e m), MonadException m, Error e) => MonadException (ErrorT e m)
(Monad (RWST r w s m), Monoid w, MonadException m) => MonadException (RWST r w s m)
(Monad (RWST r w s m), Monoid w, MonadException m) => MonadException (RWST r w s m)

onException Source

Arguments

:: MonadException m
=> m a	The computation to run
-> m b	Computation to run if an exception is raised
-> m a

If an exception is raised by the computation, then perform a final action and re-raise the exception.

class (MonadIO m, MonadException m) => MonadAsyncException m whereSource

Methods

mask :: ((forall a. m a -> m a) -> m b) -> m bSource

Executes a computation with asynchronous exceptions masked. The argument passed to mask is a function that takes as its argument another function, which can be used to restore the prevailing masking state within the context of the masked computation.

Instances

MonadAsyncException IO
(MonadIO (MaybeT m), MonadException (MaybeT m), MonadAsyncException m) => MonadAsyncException (MaybeT m)
(MonadIO (ListT m), MonadException (ListT m), MonadAsyncException m) => MonadAsyncException (ListT m)
(MonadIO (IdentityT m), MonadException (IdentityT m), MonadAsyncException m) => MonadAsyncException (IdentityT m)
(MonadIO (ExceptionT m), MonadException (ExceptionT m), MonadAsyncException m) => MonadAsyncException (ExceptionT m)
(MonadIO (WriterT w m), MonadException (WriterT w m), Monoid w, MonadAsyncException m) => MonadAsyncException (WriterT w m)
(MonadIO (WriterT w m), MonadException (WriterT w m), Monoid w, MonadAsyncException m) => MonadAsyncException (WriterT w m)
(MonadIO (StateT s m), MonadException (StateT s m), MonadAsyncException m) => MonadAsyncException (StateT s m)
(MonadIO (StateT s m), MonadException (StateT s m), MonadAsyncException m) => MonadAsyncException (StateT s m)
(MonadIO (ReaderT r m), MonadException (ReaderT r m), MonadAsyncException m) => MonadAsyncException (ReaderT r m)
(MonadIO (ErrorT e m), MonadException (ErrorT e m), MonadAsyncException m, Error e) => MonadAsyncException (ErrorT e m)
(MonadIO (RWST r w s m), MonadException (RWST r w s m), Monoid w, MonadAsyncException m) => MonadAsyncException (RWST r w s m)
(MonadIO (RWST r w s m), MonadException (RWST r w s m), Monoid w, MonadAsyncException m) => MonadAsyncException (RWST r w s m)

bracket Source

Arguments

:: MonadAsyncException m
=> m a	computation to run first ("acquire resource")
-> (a -> m b)	computation to run last ("release resource")
-> (a -> m c)	computation to run in-between
-> m c

When you want to acquire a resource, do some work with it, and then release the resource, it is a good idea to use bracket, because bracket will install the necessary exception handler to release the resource in the event that an exception is raised during the computation. If an exception is raised, then bracket will re-raise the exception (after performing the release).

bracket_ :: MonadAsyncException m => m a -> m b -> m c -> m cSource

A variant of bracket where the return value from the first computation is not required.

newtype ExceptionT m a Source

Constructors

ExceptionT
Fields runExceptionT :: m (Either SomeException a)

Instances

MonadTrans ExceptionT
Monad m => Monad (ExceptionT m)
Functor m => Functor (ExceptionT m)
(Monad (ExceptionT m), MonadFix m) => MonadFix (ExceptionT m)
(Monad (ExceptionT m), Monad m) => MonadPlus (ExceptionT m)
(Functor (ExceptionT m), Functor m, Monad m) => Applicative (ExceptionT m)
(Applicative (ExceptionT m), Functor m, Monad m) => Alternative (ExceptionT m)
(Monad (ExceptionT m), MonadIO m) => MonadIO (ExceptionT m)
(MonadIO (ExceptionT m), MonadException (ExceptionT m), MonadAsyncException m) => MonadAsyncException (ExceptionT m)
(Monad (ExceptionT m), Monad m) => MonadException (ExceptionT m)

mapExceptionT :: (m (Either SomeException a) -> n (Either SomeException b)) -> ExceptionT m a -> ExceptionT n bSource

liftException :: MonadException m => Either SomeException a -> m aSource

Lift the result of running a computation in a monad transformed by ExceptionT into another monad that supports exceptions.

Exception PatternMatchFail
Exception RecSelError
Exception RecConError
Exception RecUpdError
Exception NoMethodError
Exception NonTermination
Exception NestedAtomically
Exception BlockedIndefinitelyOnMVar
Exception BlockedIndefinitelyOnSTM
Exception Deadlock
Exception AssertionFailed
Exception AsyncException
Exception ArrayException
Exception ExitCode
Exception IOException
Exception SomeException
Exception ErrorCall
Exception ArithException